no idea when I ovulate

So I have been attempting to track when I ovulate with OPKs for the past 6 months. I usually will start around day 10 and continue until at least day 16. Out of the past 6 months I have only been able to detect 2 positives (or what I thought were positives) and at those months we BD like crazy during that time. Needless to say I am without child after all of it. Every other month I have never detected anything near another positive. I have been using cheap tests from Amazon. I myself have come to the conclusion that MAYBE I don't ovulate every month. How am I to know?? Or is it possible to not detect a positive reading but still be ovulating?? Like today which is day 19 and I feel some low cramps but I did a test real quick thinking maybe but there was nothing. HELP!!!! Anyone else experience this?